Rainbow jizz was originally thought up to be the coolest thing ever,
and to outdo purple jizz.
It is often used to burn someone really bad, but can also be used as a complement.
If you're trying to pick a chick, it works pretty well, also.

burn:
Gina:"how about that guy?"
Kaila:"ewww! Fuck no, i wouldn't fuck that guy if he had rainbow jizz."